Disclosed herein are a sweetpotato expansin cDNA (IbExpansin), a plant transformation vector carrying the same, and a transgenic plant comprising the vector. The transgenic Arabidopsis, prepared using the IbExpansin cDNA, outgrows the wild-type, thereby increasing in size and seed production up to three times. Thus, the gene is useful in the generation of highly productive transgenic plants for improvement in biomass and/or seed production.